1. Field of the Invention
The present invention relates, in general, to calibration of electronic instruments, and, more particularly, to a method and system for automatically calibrating digital carbon monoxide sensors.
2. Statement of the Problem
Carbon monoxide monitors are an increasingly popular consumer safety device. Unlike common smoke detectors, burglar alarms, and even simple carbon monoxide detectors, a carbon monoxide monitor senses and reports the value of carbon monoxide concentration rather than merely triggering an alarm once a hazard condition exists. Thus, the carbon monoxide monitor is more akin to a measurement device that the common smoke detector or burglar alarm. As in any measurement device, calibration is critical so that accurate values are reported to the user.
Carbon monoxide monitors continuously measure levels of carbon monoxide in the environment. Sources of carbon monoxide include automobiles, fireplaces, gas appliances, and other devices that combust hydrocarbon fuels. Because carbon monoxide cannot be detected by smell or taste, accurate detection is important.
Unfortunately, moderate background levels of carbon monoxide exist in most environments. To distinguish harmful levels of carbon monoxide from the background, it is necessary to have an accurate measurement of the carbon monoxide level. This measurement is typically represented in parts per million (ppm). Acceptable background levels may be as high as 25-35 ppm. Harmful exposure occurs, however, at only slightly higher concentrations of 75-100 ppm over a period of a few hours. Hence, accurate measurements of carbon monoxide concentration are critical to the task of distinguishing background CO levels from harmful exposure levels.
Carbon monoxide monitors are based around some form of sensor that produces an electrical signal (i.e., a change in resistance, capacitance, oscillation frequency, optical reflectance, transmission, or the like) in response to changes in carbon monoxide concentration. A variety of CO sensors are known and are commercially available. Sensors vary significantly from unit-to-unit in their sensitivity and offset properties. Sensitivity refers to the rate of change in output signal in response to changing levels of carbon monoxide. Offset refers to the sensor output signal when no carbon monoxide is present. In order to accurately measure CO using these sensors, the unit-to-unit differences must be compensated for in a process of calibrating the carbon monoxide monitor in which the CO sensors are installed.
Carbon monoxide monitors have traditionally been used in industrial environments. Because of this, they were low volume, relatively expensive products. Because they were not mass-marketed in the consumer marketplace, manufacturers had the luxury of calibrating each device manually since the high cost of manual calibration could be passed on to the purchaser. Prior calibration processes involved placing the CO monitor in an environment having a known concentration of CO (i.e., 100 ppm), and adjusting the monitor until it generated a correct output signal. The adjustments typically involved varying resistor or capacitor values connected to the CO sensor, or by changing the gain of amplifiers coupled to read the sensor output signal.
Another problem with manual calibration is that is often cost prohibitive to recalibrate the CO monitor as the circuits age. CO sensors often change in sensitivity and offset over time resulting in degraded performance with age. Manual calibration costs are a significant portion of a consumer CO monitor that sells for $100 or less. Occasionally calibration must be performed more than one time to rework monitors during manufacture, further increasing the cost of manual calibration. What is needed is an automatic method for calibrating carbon monoxide monitors that requires minimal human intervention to achieve accurate, reproducible results.
3. Solution to the Problem
The above problems are solved by a carbon monoxide monitor with an integrated automatic calibration circuit. Because the calibration circuit allows the carbon monoxide monitor to calibrate itself when placed in an environment with a known concentration of carbon monoxide, manual involvement in the calibration process is substantially eliminated. By limiting the cost of calibration, less expensive CO monitors are available and CO monitors can be cost effectively recalibrated as they age. Further, automatic calibration results in a level of calibration accuracy difficult to achieve with manual calibration in a production environment.